In recent years, incorporating a chemical substance into a solid material and gradually or controllably releasing the chemical substance from the inside to the outside of the solid material, a so-called “controlled release system” “sustained release system”, is regarded as noteworthy. The controlled release system is intended to provide a supply of various chemical substances having different functions in a necessary amount only when needed. The controlled release system is useful in various industrial fields, such as medicines, agricultural chemicals, cosmetics, catalysts, fertilizers and aroma chemicals. In addition to providing efficient use of chemical substances, the controlled release function is directly related to leading-edge technologies, such as technologies for reducing environmental risks (chemical pollution) and technologies for suppressing side effects in medical care (drug delivery system).
While various techniques have been proposed for a controlled release system, most of them are directed to mixing a chemical substance into a polymer gel or forming a complex consisting of a chemical substance and a polymeric material. These techniques cannot disable the continuous release of a chemical substance, even if they can reduce the rate of the controlled-release of the chemical substance. That is, they cannot provide an on-off control function of controllably releasing a chemical substance in a necessary amount only when needed.
There have also been proposed some controlled release systems having an on-off control function based on a device using an electrode (Japanese Patent Laid-Open Publication Nos. H 05-269373, H 05-261278, H 05-231560, H 05-221469, H 05-221468 and H 05-212277). Since these systems are essentially provided with a power source, an electrode and an interconnection therebetween, to allow a chemical substance to be controllably released according to an electric field signal, they are not applicable in a specific region where the above interconnection cannot be constructed, for example, within a living body. Thus, there is a need for a controlled release system having an on-off control function in the form of a micro self-sustained system independent of a power source and others.
Silica (gel) is widely used because it has almost no harm to the environment and living organisms. As one of features, silica has large pores capable of absorbing and incorporating various kinds of chemical substances therein. While it is contemplated to use silica (gel) in a controlled release system in such a manner that a chemical substance incorporated therein is controllably released therefrom, no on-off control function will be attained if silica (gel) is used as it is.
A technique for providing a controlled-release on-off control function to silica gel or capsule-type silica has been proposed recently (Japanese Patent Laid-Open Publication Nos. 2000-279817, 2001-131249 and 2001-213992). However, an accurate controlled-release control of an incorporated substance cannot be achieved due to uneven size in pores of the silica gel. Thus, there is a need for a material with pores having more uniform outlets or access regions to the outside.
A material fulfilling the above requirement includes hexagonal mesoporous silica, such as MCM-41, having pores with regularity in diameter and configuration. There is a recent report disclosing a spontaneous-diffusion controlled-release property of a drug incorporated in MCM-41 (M. Vallet-Regi et al, Chem. Mater. Vol. 13, p 308, 2001; B. Muñoz et al., Chem. Mater. Vol. 15, p 500, 2003). This report includes no description of controlled-release on-off control.